Cambrian Period

Geologic Map of Missouri, 1990, Missouri Department of Natural Resources, Division of Geology and Land Survey (now Geological Survey and Resource Assessment Division).

Cambrian Period (544 to 505 million year old rocks)

Cambrian Rocks record the first time Missouri was covered by a shallow continental sea in the Phanerozoic. By studying the first layers of Missouri Cambrian rocks (usually considered the Lamotte Sandstone), geologists can determine that the highest igneous mountains probably were not underwater, but that the mountain range (and possibly sediments from the Transcontinental Arch to the west) furnished the weathered rock for the sandstone.

All Missouri rocks of Cambrian Age or younger (save a few ash beds, pipelike igneous structures called diatremes and glacial deposits from elsewhere) are sedimentary-- that is they consist of fragments or chemicals derived from other rocks after weathering transport, deposition and lithification. This continental flooding is a by-product of world-wide sea level rise. This rise, geologists know, starts at continental margins and works its way inland. Missouri was not covered by water until halfway through the Cambrian Period, during a time called the Sauk transgression sequence, according to a system set up by Lawrence Sloss in the mid 1960's. These transgression sequences record North American geological events more closely than the European period system. Since geology is a worldwide science, it is good to be familiar with both.

Because of continental movement, Most of North America was very near the earth's equator during the Cambrian. These warm continental seas were never very deep, a couple hundred meters at most. Nearly all Missouri sediments were deposited under shallow conditions.

Another factor to remember is that in the Paleozoic Era (544 to 286 million years ago) North America had not yet attained its familiar shape or topography. During the hundreds of millions of years when sedimentary Missouri was being deposited, it is quite likely that the "the ocean" was only a few hundred miles to the south of the border--about mid-Arkansas at the most. Determining ocean currents on the basis of rock strata can be done--and most indicate that our interaction with the deep ocean came generally from the south (once the water got around the St. Francois Mountains somehow).

Between the Proterozoic volcanism and Cambrian deposition was nearly a billion years of time. In southern Missouri, this great time gap can be bridged with a human hand. When such an erosion surface occurs, followed by new deposits, it is called an unconformity.

The first rock unit, the Lamotte Sandstone, consists of cemented rock fragments, most likely washed out to sea from the newly inundated continents. Angular quartz, feldspar, and dark minerals are all in the lower levels of the Lamotte. As you go up in the rock column, the sandstone gets "cleaner"--that is, is consists of more rounded quartz only, and fewer rock fragments.

Not all Missouri Cambrian rocks are sandstone. Above the sandstone, one finds the Bonne Terre Formation (limestone/dolomite, sands and shale), the Davis Formation (known for its dark shale and rip-rappy conglomerate), and the Derby-Doe Run, Potosi and Eminence Dolomites. These secondary dolomites (secondary, meaning they were laid down as limestone, and later more or less changed to dolomite) can be hundreds of feet thick in places. What this change in rock type means is that as time progressed, the seas got deeper, as sandstones generally indicate land or beach deposits, carbonates forming as shallow marine reefs or precipitates. Shales can be either very shallow tidal flat deposits, or very deep sea mud, depending on their character and fossils.

Cambrian fossils in Missouri are largely trilobites and brachiopods. Shelly animals had just developed. Stromatolites (structures made by cyanobacteria, and stromatolitic reefs (sometimes converted to chert) also occur here. Unfortunately, when limestone becomes dolomitized, fossils are often erased. In Missouri, many fossils which survive became chertified before the dolomitization occurred, and those remain for us to see.

Cambrian rock layers which are economically important are the Lamotte, Bonne Terre and Derby-Doe Run. These three layers are the primary source for galena, the state mineral, in southern Missouri. For more on galena deposits, see the Pennsylvanian page.

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